Allow for nullable collection properties

[ibsh]

Summary of Changes

If a to-many relationship property is nullable, the current CoreDataGeneratedAccessors template generates multiple nullability build errors when using -Wnullable-to-nonnull-conversion. This change is a naive fix for those errors but may be appropriate unless more detailed consideration of nullable properties is warranted.

Addresses

Partially addresses #336

If, during CoreDataGeneratedAccessors, a relationship property is null,
fall back to a newly instantiated NSOrderedSet.

[sixten]

There appears to be a syntax error in the updated templates: each of the edited lines is missing a closing square bracket.